That would be a standard reversing of a series or compound wound motor, but inwo says it works without out it and it is a small terminal marked A.
What about for an ammeter and is uses the field as a shunt?
Max.

I found a winch control box circuit earlier. I didn't take the time to understand it, i should have copied the link. Sorry. It think it was just a reference to keep the motor chassis at ground (neg battery terminal reference) on trucks, boats or other 12 or 24 volt circuits powering winches.

A is normally for "Armature". BillB3857's schematic is for a reversable motor where the two field wires are reversed, and the main armature current is the A winding.

In Inwo's motor the small "A" terminal might be for armature grounding, a grounding brush. Maybe to stop potential problems in winch motors that could be caused by the spooling of plastic rope etc building up static on the armature?

If it is a grounding brush it just connects to the armature steel shaft, and will not produce any voltage at that terminal when the motor is spinning (except some noise).